The present invention relates generally to magnetic resonance imaging, and more specifically, to a system and method designed to reduce the effects of magnetization transfer on image quality and resolution. By transmitting tailored RF pulses according to particular k-space trajectories, the transfer of magnetization from excited nuclei in a desired slice to nuclei of neighboring slices may be reduced.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, MZ, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
One factor affecting the strength and/or signal to noise ratio (SNR) of these NMR signals is known as “magnetization transfer.” Magnetization transfer is the exchange of magnetization between macromolecular or “bound” protons and bulk or “free” protons, most prevalent in water. This exchange can occur, for example, by dipolar interaction or chemical exchange between different tissue types and molecules. Magnetization transfer is generally proportional to transmit power, though it also depends upon other conditions. Since the T2 times of bound protons are generally much shorter than the T2 times of free protons, it can be difficult to directly acquire MR signals from bound protons. Therefore, some processes, typically referred to as “magnetization transfer imaging” (MTI) techniques, use the degree of interaction between bound protons and free protons as an image weighting.
Frequently, however, magnetization transfer can have an adverse effect on image quality and/or SNR of non-MTI imaging. When an RF pulse is applied at a resonant frequency for a given slice, nearby off-resonant bound protons can become saturated due to magnetization transfer. The extent of saturation is dependent upon several factors including the type of tissue of interest, the T1 of the tissue of interest, the T2 of nearby bound spins, and the rate of exchange of magnetization therebetween, but often extends about 10 kHz on either side of the selected slice. The saturation of the off-resonant bound protons can then transfer to free protons of the slice to be imaged, further affecting signal quality. Conversely, when tissues having many bound protons are to be imaged (such as white matter), magnetization transfer can reduce the amount of net magnetization in the tissue(s) of interest and affect the imageability thereof.
One type of pulse which may limit transmit power (SAR), as compared to a conventional sinc pulse, is known as a variable rate selective excitation (VERSE) pulse. VERSE pulses are typically employed as a technique for reducing peak and total power over a high amplitude portion of a pulse sequence. These pulses are usually derived from an RF pulse conventionally shaped for a desired flip angle, duration, and bandwidth. However, the higher amplitude portions of the pulse are then reshaped to reduce peak and total power. As shown in FIG. 1, a VERSE pulse 2 is generally characterized by a lengthened or stretched main lobe 4 that is significantly longer than other lobes 6 of the pulse. Although the RF power is reduced, it is spread over a larger region. As a result, though the MT effect is reduced over the 10 kHz on either side of the slice select location, there are more spins being affected and thus more aggregate MT within that band. During the stretched mainlobe 4, the slice select gradient 8 is reduced. This has the effect of spreading the MT effect over a wider extent in the slice select direction, acting to increase the cumulative MT effect in a multi-slice sequence. This spreading effect mitigates the previously mentioned lowering of MT effects. Thus while VERSE pulses lower SAR, they are still prone to producing MT effects in multi-slice acquisitions, and may therefore produce poor signal quality in certain circumstances.
It would therefore be desirable to have a system and method capable of reducing the occurrence of off-resonance magnetization transfer saturation while maintaining a high signal quality and acceptable specific absorption rate (SAR) for MR imaging.